Bulk-heterojunction (BHJ) polymer solar cells (PSCs) based on interpenetrating polymer donor and fullerene acceptor networks offer a compelling approach to efficient solar energy harvesting at low costs. While PSC performance has advanced steadily, with 6-8% single junction power conversion efficiencies (PCEs) reported in the peer-reviewed literature, these metrics still lag behind those of inorganic cells. PCE (ηe) is a key index of photovoltaic device performance, and is the product of the short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (FF), where Pin is the incident solar power (equation 1):
                              η          e                =                                            V              oc                        *                          J              sc                        *            FF                                P                          i              ⁢                                                          ⁢              n                                                          (        1        )            To maximize PCE, research efforts have focused on developing narrow bandgap polymers to maximize solar photon capture and increase Jsc, while lowering the polymer highest occupied molecular orbital (HOMO) to increase Voc and environmental stability. Nevertheless, PSC power conversion efficiencies (PCEs) are still constrained by low fill factors (FFs), typically below 70%. The realization of high FFs has proven elusive, although there is evidence that carrier mobility, active layer microstructure, and interfacial charge recombination are factors. Mobilities in turn are sensitive to the film morphology, with order typically enhancing transport. However, highly crystalline polymers can also exhibit deleterious aggregation, poor solubility, and non-optimal phase separation in PCBM blends.
Accordingly, the art desires new polymers for PSCs, particularly those that can confer a high fill factor.